1. Field of the Invention
This invention relates to an optical recording medium of phase change type and a method for recording such an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is a phase change type optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline states whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information.
The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit used for their operation is simple.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of the power (recording power) such that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of the power (erasing power) such that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature is applied. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single laser beam.
In recent years, special attention is being given to rewritable digital video discs (DVD-RAM) wherein a recording film of phase change type is employed. In DVD-RAM Ver. 1.0, data of 2.6 GB is recorded on one side of the disc having a diameter of 120 mm. The recording is conducted in such instance at a recording wevelength of 0.65 .mu.m, a numerical aperture NA of the optical pick up of 0.60, and a track pitch of 0.74 .mu.m and with the recording system of land/groove recording and modulation system of 8-16 modulation, and the minimum mark length is 0.62 .mu.m. Data transfer rate of 11.06 Mbps is attained by adopting the relative linear velocity of the disc in relation to the optical pick up of 6 m/s.
Jpn. J. Appl. Phys. vol.37 (1998) pp.2104-2110 describes ZnS--SiO.sub.2 (100 nm)/interface layer (5 nm)/Ge.sub.2 Sb.sub.2 Te.sub.5 (20 nm)/ZnS--SiO.sub.2 (20 nm)/Al Alloy (150 nm) as a film structure by taking the DVD-RAM specification into consideration. It should be noted that the number in the brackets is thickness.
In an optical recording medium of phase change type, recording at a higher recording density and a higher linear velocity is realized by improvements either on the side of the media drive system or on the side of the medium itself. Improvements of the drive system include shortening of the wavelength of the recording laser beam, increase of NA of the optical pick up, and the like, and improvements of the medium include narrowing of the track pitch, shortening of the minimum mark, and the like. Of the improvement on the side of the medium, the narrowing of the track pitch may result in an increased cross erase wherein the record marks on the adjacent track are erased, an increased cross talk wherein the record marks on the adjacent track are read out, and a reduced C/N. Shortening of the minimum mark invites decrease in C/N as well as thermal and waveform interference between the adjacent marks to result in fluctuation in the mark length and increase in the jitter.
In consideration of such situation, the inventors of the present invention have conducted a recording by using the film structure described in Jpn. J. Appl. Phys. vol.37 (1998) pp. 2104-2110 at a recording wavelength of 0.65 .mu.m and an NA of 0.60 and using the 8-16 modulation system and the land/groove recording as in the case of DVD-RAM, and at a minimum mark length (0.42 .mu.m) and a track pitch (0.60 .mu.m) which are shorter than those of DVD-RAM. The jitter properties were unfavorable.
Japanese Patent Application Kokai No. (JP-A) 225603/1993 describes a film structure for use in the high density recording. This film structure comprises a transparent substrate/an undercoat layer/a recording layer/an overcoat layer/a metal reflective layer. The recording layer is a film of a GeSbTe alloy having a thickness of 15 to 30 nm or 30 to 45 nm or 70 to 110 nm. The undercoat layer and the overcoat layer are films of a transparent dielectric material having a refractive index of 1.9 to 2.2, and the undercoat layer is deposited to a thickness of 160 to 220 nm and the overcoat layer is deposited to a thickness of 80 to 160 nm or 200 to 280 nm. This film structure is used for the optical recording at a wavelength of 450 to 670 nm, and there is described in JP-A 225603/1993 that the film structure adopted therein resulted in an increased absorption of the laser beam as well as an increased contrast between the crystalline state and the amorphous state and that a favorable reproduction/erasure of the recorded information was thereby realized. In an Example of JP-A 225603/1993, the mark length at the critical recording density is 0.7 .mu.m at the recording wavelength of 457.9 nm, and no description is included for the reproduction properties upon recording of the shorter record marks.
In consideration of such situation, the inventors of the present invention have conducted a recording by using the film structure of ZnS--SiO.sub.2 (160 nm)/Ge.sub.2 Sb.sub.2 Te.sub.5 (16 nm)/ZnS--SiO.sub.2 (80 nm)/Al Alloy at a recording wavelength of 0.65 .mu.m and an NA of 0.60 and using the 8-16 modulation system and the land/groove recording as in the case of DVD-RAM, and at a minimum mark length (0.42 .mu.m) and a track pitch (0.60 .mu.m) which are shorter than those of DVD-RAM. The jitter properties were unfavorable.
In optical disc systems such as DVD-RAM wherein the recording and the reproduction are conducted by mark edge detecting system using a recording film of phase change type, it is quite important to prevent deformation of the mark shape and insufficient erasure of the record marks. For this, record marks should be formed such that the temperature reached and the cooling rate will be substantially the same at every position in the periphery of the region where the recording film is melted.
In correspondence with the speedup in the digital signal processing, there is a strong demand for a faster recording and reproduction in the information recording system. To meet such demand, increase in the data transfer rate is sought by increasing the relative velocity of the information recording medium in relation to the laser beam. In view of such situation, an information recording method capable of preventing the mark shape deformation and the insufficient record mark erasure are highly awaited.